now we offer also a generic version that can work with any SIP server. You can buy a license from our web, I cannot post a link here but go to voipswitch main page and there more products, retail mobile clients and you will find it. it works over wifi and 3G as well
There are also voip clients for symbian, android and windows mobile.
Since everyone seems to have a SIP client these days, here is ours:
shop.fgmicrotec.com
works on:
OS 4.6 = 8520, 8900 and 9000
OS 5.0 = 8520, 9000 and 9700
If what you have tried didn't quite meet your expectations you might want to give fgVoIP a try . We think it works very well but obviously it is the user who decides.
In the mean time we encourage everyone to try the client and let us know what you think.